In the present time, to eliminate noise initiatively, people invented the technology called ‘active noise reduction’, wherein the principle is that it is well known all the sounds consist of a certain frequency spectrum. If we can find a sound whose frequency spectrum is totally the same with the noise to be eliminated but just has an opposite phase position (phase difference of 180°), we can totally counteract the noise. The key of the technology is how to find out the sound to counteract the noise. The actual solution is based on the noise source itself, reversing the phase position of the original noise through electronic circuit.
Now more and more earphones apply the active noise reduction technology. The technology is divided into digital noise reduction and analog noise reduction. For the analog noise reduction, how the filter circuit frequency response curve fits the earphone acoustic frequency response curve is the key factor to influence the effect of noise reduction. The noise reduction circuit of the initiative noise reduction earphone applies the above mentioned discrete components or applies the fitted noise reduction chip incorporating with outer circuit to realize the noise reduction. The situation of narrower range of noise reduction bandwidth sometimes exists, wherein the noise reduction bandwidth is about 100 HZ to 500 HZ. Thus it can not achieve a wider frequency band range of noise reduction, so that the effect of noise reduction is not very good and the fitting degree of frequency response curve is not very high.